How to Save a Dying Rhododendron (In 4 Steps)

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Many homeowners out there love adding rhododendrons and azaleas to add bursts of color to their gardens throughout summer and spring.

It’s even better with rhododendrons as they have large, bell-shaped heads at the end of each stem, and each stem holds ten or more stamens.

Moreover, rhododendrons are often labeled as easy-to-care-for plants, which they are, but only when they’re planted in ideal conditions. If anything is off with the growing conditions, this is one shrub that’s not very forgiving!

That’s why today, I’ll share with you some of the most vital parts of caring for rhododendrons as well as teach you how to save dying rhododendrons in four steps! 

So, put on your gardening gloves, and let’s start!

Rhododendron Growing Checklist

1. Soil Acidity

Rhododendrons need acidic soil with a pH of 5.5 or less, so you’ll need to adjust if it’s unsuitable. One thing to watch out for is that some methods of acidifying soil to try to save a dying rhododendron can kill it. 

For instance, aluminum sulfate is one of the most harmful as it’s toxic to the roots. Lime can be damaging as well.

Also, if you plant your rhododendron too close to paved pathways or a building, there could be lime leaching into the soil, increasing the pH levels.

The best practice for planting rhododendrons is planting them at least 5 feet from a building’s foundations. However, it’s safer to grow it at least 5 feet away from any other structure that could cause lime to leach and alter the soil acidity.

If you have soil with a pH over 7.5, it’ll be incredibly challenging to lower it substantially. In this case, container-growing rhododendrons would be the easier alternative, bypassing a lot of guesswork.

2. Drainage

For rhododendrons, you need to use porous soil that’s well-draining. You can test your soil drainage by digging a 12” x 12” hole, filling it with water, letting it drain, then refilling it.

Time the second time you fill the hole with water. It should drain by about one inch per hour. If yours is draining faster, it’s likely too dry. Hence, it would benefit from more organic material to improve moisture retention. 

On the other hand, if you’re having too slow drainage, it means the roots would be sitting in standing water, putting them at risk of root rot. The answer here would be to mix some fine gravel or other suitable material to increase the soil’s porosity.

For those growing rhododendrons in a container, you can add up to 20% perlite or pumice to improve drainage.

3. Sunlight and Shade Requirements

Rhododendrons are a bit tricky when it comes to their sun requirements. This is mainly because they need a delicate balance between sunlight and shade. Essentially, they thrive in environments that mimic the dappled light of forests.

Of course, that can be rather rough to replicate in your backyard! So, here’s what you need to do:

In the early morning, let your flowers enjoy as much of the sun as you can. There’s no need to shield them from it before noon. 

However, as soon as the afternoon rolls around, you need to shade your flowers, which you can do through various means. 

For instance, if you can plant your rhododendrons next to a tree, that’ll be excellent. The leaves and branches will keep the sun from scorching the delicate petals.

You can also use a pretty shade cloth, which you can place strategically to protect the flowers from the sun during its peak hours. 

4. Watering Schedule

Rhododendrons are the type of plants that like extra attention when it comes to water. It’s because they’re shallow-rooted plants that can quickly be affected by both overwatering and underwatering.

Usually, the best watering schedule for growing rhododendrons is around twice weekly. Once they’re healthy and grown, you can start watering them only during dry periods, which means once every two weeks.

Use a soil moisture sensor to help you check whether the rhododendrons need water or not. Also, when watering keep your aim at the base of the plant to avoid wetting the leaves, which can promote fungal issues.

5. Nutrient Competing Plants

As mentioned, rhododendrons have shallow roots, and other plants and trees can easily over-compete them.

Because they need dappled sunlight, the solution is often to plant it under a tree so it has some canopy to shade it from the hot afternoon sun.

That can work unless it’s a maple tree, which has shallow roots, too, so those will compete with rhododendrons for nutrients.

Walnut trees are worse as they produce toxins that can kill rhododendrons, so if you have yours planted near a walnut tree, you’ll need to remove it.

Rhododendrons grow best with four to six feet of space between them and other plants with similar shallow roots. 

If your design calls for companion plants, choose plants with a deeper root system that won’t be competing for the same nutrients in the topsoil around rhododendrons.

Checklist Summary

• Soil acidity: Under 5.5 pH (if the pH is above 7.5, grow in a container, or a raised garden bed).
• Drainage: Porous soil that drains at an average of 1” per hour. Soil amendments may be needed.
• Sunlight: A delicate balance that offers gentle sunlight and shade at the sun’s peak.
• Watering: water around once or twice a week while growing and during every drying period.
• Surrounding plants shouldn’t have shallow roots as those compete for nutrients in the soil.
• Rhododendrons transplanted in the ground should be at least five feet from buildings and other structures.
• If you’re growing more than one rhododendron, put 4 to 6 feet of space between them to avoid restricting air circulation.

Finally, most problems that affect rhododendrons are because one or more of their preferred growing conditions aren’t being met.

For example, early signs of root rot are likely to show as wilting and yellowing on the leaves due to improper soil drainage. 

Likewise, fungal infections are encouraged when there’s insufficient airflow caused by grouping two or more rhododendrons or other shallow-rooted plants too close.

So, check your growing conditions against the previous checklist to see if you need to move the plant, or if there’s anything you can do to fix the soil.

6 Common Diseases That Affect Rhododendrons

Several fungal issues affect rhododendrons when the growing environment isn’t ideal, such as:

1. Bud Blast

Sadly, Bud Blast makes your flowers look terrible. They turn brown and dry up, and by next spring, when you expect blossoms, you wind up with bristly spores instead!

Bud blast is a fungal infection that kills flower buds, and if left untreated, it can spread to branches and travel throughout the plant. What causes it? It’s mainly leaf hoppers.

The buds on rhododendrons have slits that leafhoppers use to lay their eggs. When they do, they introduce a fungal infection. When you have an infection on your hand, you’ll need to remove the dead flowers and dry branches around the damaged ones to start again.

To keep the grasshoppers away, you can apply a coating of diatomaceous earth around the flowers, especially in spring and summer.

2. Leaf Gall / Flower Gall

Rarely will you find a growth-like tumor on a plant to be good news. Surprise! It is in this case! Leaf gall just looks super unhealthy on any plant or infected flower, but it won’t necessarily harm a rhododendron. 

To explain, it’s caused by a fungus called Exobasidium vaccinii. In its early stages, the leaf turns a paler green color, and later, as the weather gets more humid, the infected leaf swells and develops a white coating over all or part of the leaf.

Those are fungal spores that spread! The spores can overwinter and re-emerge next spring, so any infected parts should be trashed. Don’t compost any leaves or flowers with galls.

3. Petal Blight

When your rhododendrons develop freckles on their petals, it’s probably petal blight caused by ovulinia fungus. Unfortunately, this is a tough one! 

Early stages show brown freckles on white petals and white spots on colored petals. At first, these little spots might not seem like much. However, give them time, and they’ll puff up, turn the flower mushy, and cling to the leaves as well!

Before you know it, the fungus will take over. If caught early enough, you can nip it in the bud by discarding infected parts of the plant. 

However, The fungus will survive in the soil, so it’s best you replace ground mulches with fresh organic materials. If you’re growing in containers, replace the top layer of potting soil.

Once a rhododendron becomes a host plant, you’ll need to take extra care to prevent it. Do that by keeping a layer of mulch about 4” thick and thin near the trunk.

If it does become an ongoing problem, treat it with a fungicide before buds start opening using a product that contains chlorothalonil, or captan. 

4 – Dieback and Root Rot caused by Phytophthora

Phytophthora cinnamomi causes root rot, and phytophthora cactorum causes dieback. They’re both soil-borne pathogens that can lay in dormancy for years. Then, when the conditions are just right, they spring into action and kill the roots of everything in their path.

Phytophthora root rot can kill a rhododendron in under two weeks. Early signs of dieback include chlorosis (yellowing leaves), leaf curl, lesions on the leaves, and leaf curling inward before eventually dropping.

Symptoms of root rot caused by Phytophthora are much more challenging to spot. This is because the pathogen enters through finer roots, spreading to the trunk and the rest of the shrub.

Remember, mature rhododendrons can show gradual signs of wilting, drooping, and chlorosis for up to a year before dying. The only way to tell if your rhododendron has phytophthora in the soil is to have the soil tested.

Also, phytophthora needs specific conditions to become a threat. Those are moist, acidic soil that drains poorly. Four hours of standing water is all it takes for phytophthora to germinate!

Fix the soil conditions, and next time, try growing a phytophthora-resistant rhododendron!

5. Cankers Caused by Rhizoctonia or Botryosphaeria

Two main types of cankers can affect rhododendrons, and those are caused by Rhizoctonia or botryosphaeria. 

Rhizoctonia 

Rhizoctonia web blight is mainly visible on the leaves when humidity and temperatures are high. By looking at the brown areas on an infected plant’s leaves, you’ll see a brown webbing covering them. 

That’s the rhizoctonia fungus. The leaves mat tightly together, holding them under the canopy, then when temperatures drop below 70°F (12°C), they fall off.

Take a look at the roots of the plant near the soil line for brown lesions or cankers, to confirm a rhizoctonia fungal infection

Rhizoctonia infections start near the soil line with a brown or reddish-brown lesion that eventually enlarges, creating a canker.

A canker grows and can girdle around the stem/trunk restricting nutrient uptake from the soil. The drought effect is most noticeable with wilting midday when temperatures are higher.

Rhizoctonia is soil-borne, but unlike Phytophthora, it isn’t quick to kill rhododendrons. It’s harmful, but not fatal if treated with a fungicide. Pruning some branches, twigs, and leaves to increase airflow will help, too.

Botryosphaeria 

Botryosphaeria cankers have similar symptoms, but unlike rhizoctonia, web blight on the leaves won’t be present. Instead, leaves will wilt and die, as will parts of the branches and twigs.

Botryosphaeria enters wood through wounds in the bark, especially with cuts made with tools that weren’t disinfected to prevent infections from other plants.

The only way to save a rhododendron from Botryosphaeria cankers is to cut away dead and dying tissue late into its dormancy period but shortly before the buds open. In most species, that’s early spring.

The severity of infection will determine how much pruning needs to be done. If it’s severe with multiple dying branches causing poor foliage, it can be cut down to 6” above ground level. 

Up to 15” can be sufficient. As you’re cutting the branches back, pay attention to the main branches extending from the crown. 

There are usually more than three primary branches, then multiple off-shoots. Prune the off-shoots first as those will grow back faster than the main branches. 

If the canker is on a main branch, cut a few inches below it to make sure only healthy wood is growing back in.

How to Save a Dying Rhododendron from Most Fungal Infections in 4 Steps

Four general steps can help you bring your rhododendron from the brink of infection damage:

1. Remove any diseased foliage and dead or dying branches and twigs.
2. Apply a fungicide treatment that contains chlorothalonil, or captan.
3. Container-grown rhododendrons should have their soil changed. If transplanted in the ground, consider potting. If it’s too big to move, replace most of the soil surrounding the roots and trunk and apply a new layer of organic mulch to about a 4” thickness. 
4. Prune and deadhead to encourage new growth faster and to improve air circulation, which lowers humidity levels too. Much of the fungal infections affecting rhododendrons happen when humidity levels soar above 90%.

Phytophthora is the only fungal infection a rhododendron—or any neighboring plants—can’t be saved from. 

You can only rescue it from such a fungal infection if you catch it super early, which is unlikely as it attacks the roots and stems from the inside. 

All other fungal diseases can be treated with the removal of infected parts, and if required, repeated fungicide applications.

Final Thoughts

So, are you still wondering how to save a dying rhododendron? Hopefully not! To give you a quick recap, almost all fungal infections affecting your rhododendrons are fixable. 

However, to prevent them, you must follow a strict caring routine.

Monitor the soil pH, ensure proper drainage, manage the sunlight, and follow a consistent watering schedule. 

Also, remember to plant the flowers where they won’t have to compete with other plants for nutrients!

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